my %compressors = (
    xz => "| $Nix::Config::xz",
    bzip2 => "| $Nix::Config::bzip2",
    none => ""
);

my %compressors = ();
$compressors{"none"} = "";
if (defined($Nix::Config::bzip2)) {
    $compressors{"bzip2"} = "| $Nix::Config::bzip2",
}
if (defined($Nix::Config::xz)) {
    $compressors{"xz"} = "| $Nix::Config::xz",
}